The present invention relates to the discovery, identification and characterization of nucleotides that encode Ob receptor (ObR), a receptor protein that participates in mammalian body weight regulation. The invention encompasses obR nucleotides, host cell expression systems, ObR proteins, fusion proteins, polypeptides and peptides, antibodies to the receptor, transgenic animals that express an obR transgene, or recombinant knock-out animals that do not express the ObR, antagonists and agonists of the receptor, and other compounds that modulate obR gene expression or ObR activity that can be used for diagnosis, drug screening, clinical trial monitoring, and/or the treatment of body weight disorders, including but not limited to obesity, cachexia and anorexia.
Obesity represents the most prevalent of body weight disorders, and it is the most important nutritional disorder in the western world, with estimates of its prevalence ranging from 30% to 50% within the middle-aged population. Other body weight disorders, such as anorexia nervosa and bulimia nervosa which together affect approximately 0.2% of the female population of the western world, also pose serious health threats. Further, such disorders as anorexia and cachexia (wasting) are also prominent features of other diseases such as cancer, cystic fibrosis, and AIDS. obesity, defined as an excess of body fat relative to lean body mass, also contributes to other diseases. For example, this disorder is responsible for increased incidences of diseases such as coronary artery disease, stroke, and diabetes. (See, e.g., Nishina, P. M. et al., 1994, Metab. 43:554-558) Obesity is not merely a behavioral problem, i.e., the result of voluntary hyperphagia. Rather, the differential body composition observed between obese and normal subjects results from differences in both metabolism and neurologic/metabolic interactions. These differences seem to be, to some extent, due to differences in gene expression, and/or level of gene products or activity (Friedman, J. M. et al., 1991, Mammalian Gene 1:130-144).
The epidemiology of obesity strongly shows that the disorder exhibits inherited characteristics (Stunkard, 1990, N. Eng. J. Med. 322:1483). Moll et al. have reported that, in many populations, obesity seems to be controlled by a few genetic loci (Moll et al. 1991, Am. J. Hum. Gen. 49:1243). In addition, human twin studies strongly suggest a substantial genetic basis in the control of body weight, with estimates of heritability of 80-90% (Simopoulos, A. P. and Childs B., eds., 1989, In Genetic Variation and Nutrition in Obesity, World Review of Nutrition and Diabetes 63, S. Karger, Basel, Switzerland; Borjeson, M., 1976, Acta. Paediatr. Scand. 65:279-287).
Studies of non-obese persons who deliberately attempted to gain weight by systematically over-eating were found to be more resistant to such weight gain and able to maintain an elevated weight only by very high caloric intake. In contrast, spontaneously obese individuals are able to maintain their status with normal or only moderately elevated caloric intake. In addition, it is a commonplace experience in animal husbandry that different strains of swine, cattle, etc., have different predispositions to obesity. Studies of the genetics of human obesity and of models of animal obesity demonstrate that obesity results from complex defective regulation of both food intake, food induced energy expenditure and of the balance between lipid and lean body anabolism.
There are a number of genetic diseases in man and other species which feature obesity among their more prominent symptoms, along with, frequently, dysmorphic features and mental retardation. For example, Prader-Willi syndrome (PWS) affects approximately 1 in 20,000 live births, and involves poor neonatal muscle tone, facial and genital deformities, and generally obesity.
In addition to PWS, many other pleiotropic syndromes which include obesity as a symptom have been characterized. These syndromes are more genetically straightforward, and appear to involve autosomal recessive alleles. The diseases, which include, among others, Ahlstroem, Carpenter, Bardet-Biedl, Cohen, and Morgagni-Stewart-Monel Syndromes.
A number of models exist for the study of obesity (see, e.g., Bray, G. A., 1992, Prog. Brain Res. 93:333-341, and Bray, G. A., 1989, Amer. J. Clin. Nutr. 5:891-902). For example, animals having mutations which lead to syndromes that include obesity symptoms have been identified, and attempts have been made to utilize such animals as models for the study of obesity. The best studied animal models, to date, for genetic obesity are mice models. For reviews, see for example, Friedman, J. M. et al., 1991, Mamm. Gen. 1:130-144; Friedman, J. M. and Liebel, R. L., 1992, Cell 69:217-220.
Studies utilizing mice have confirmed that obesity is a very complex trait with a high degree of heritability.
Mutations at a number of loci have been identified which lead to obese phenotypes. These include the autosomal recessive mutations obese (ob), diabetes (db), fat (fat) and tubby (tub). In addition, the autosomal dominant mutations Yellow at the agouti locus and Adipose (Ad) have been shown to contribute to an obese phenotype.
The ob and db mutations are on chromosomes 6 and 4, respectively, but lead to a complex, clinically similar phenotype of obesity, evident starting at about one month of age, which includes hyperphagia, severe abnormalities in glucose and insulin metabolism, very poor thermoregulation and non-shivering thermogenesis, and extreme torpor and underdevelopment of the lean body mass. This complex phenotype has made it difficult to identify the primary defect attributable to the mutations (Bray G. A., et al., 1989 Amer. J. Clin. Nutr. 5:891-902).
Using molecular and classical genetic markers, the db gene has been mapped to midchromosome 4 (Friedman et al., 1991, Mamm. Gen. 1:130-144). The mutation maps to a region of the mouse genome that is syntonic with human, suggesting that, if there is a human homolog of db, it is likely to map to human chromosome 1p.
The ob gene and its human homologue have recently been cloned (Zhang, Y. et al., 1994, Nature 372:425-432). The gene appears to produce a 4.5 kb adipose tissue messenger RNA which contains a 167 amino acid open reading frame. The predicted amino acid sequence of the ob gene product indicates that it is a secreted protein and may, therefore, play a role as part of a signalling pathway from adipose tissue which may serve to regulate some aspect of body fat deposition. Further, recent studies have shown that recombinant Ob protein, also known as leptin, when exogenously administered, can at least partially correct the obesity-related phenotype exhibited by ob mice (Pelleymounter, M. A. et al., 1995, Science 269:540-543; Halalas, J. L. et al., 1995, Science 269:543-546; Campfield, L. A. et al., 1995, Science 269:546-549). Recent studies have suggested that obese humans and rodents (other than ob/ob mice) are not defective in their ability to produce ob mRNA or protein, and generally produce higher levels than lean individuals (Maffei et al., 1995, Nature Med. 1(11):1155-1161; Considine et al., 1995, J. Clin. Invest. 95(6):2986-2988; Lohnqvist et al., 1995, Nature Med. 1:950-953; Hamilton et al., 1995, Nature Med. 1:953-956). These data suggest that resistance to normal or elevated levels of Ob may be more important than inadequate Ob production in human obesity. However, the receptor for the ob gene product, thought to be expressed in the hypothalamus, remains elusive.
Homozygous mutations at either the fat or tub loci cause obesity which develops more slowly than that observed in ob and db mice (Coleman, D. L., and Eicher, E. M., 1990, J. Heredity 81:424-427), with tub obesity developing slower than that observed in fat animals. This feature of the tub obese phenotype makes the development of tub obese phenotype closest in resemblance to the manner in which obesity develops in humans. Even so, however, the obese phenotype within such animals can be characterized as massive in that animals eventually attain body weights which are nearly two times the average weight seen in normal mice.
The fat mutation has been mapped to mouse chromosome 8, while the tub mutation has been mapped to mouse chromosome 7. According to Naggert et al., the fat mutation has recently been identified (Naggert, J. K., et al., 1995, Nature Genetics 10:135-141). Specifically, the fat mutation appears to be a mutation within the Cpe locus, which encodes the carboxypeptidase (Cpe) E protein. Cpe is an exopeptidase involved in the processing of prohormones, including proinsulin.
The dominant Yellow mutation at the agouti locus, causes a pleiotropic syndrome which causes moderate adult onset obesity, a yellow coat color, and a high incidence of tumor formation (Herberg, L. and Coleman, D. L., 1977, Metabolism 26:59), and an abnormal anatomic distribution of body fat (Coleman, D. L., 1978, Diabetologia 14:141-148). This mutation may represent the only known example of a pleiotropic mutation that causes an increase, rather than a decrease, in body size. The mutation causes the widespread expression of a protein which is normally seen only in neonatal skin (Michaud, E. J. et al., 1994, Genes Devel. 8:1463-1472).
Other animal models include fa/fa (fatty) rats, which bear many similarities to the ob/ob and db/db mice, discussed above. One difference is that, while fa/fa rats are very sensitive to cold, their capacity for non-shivering thermogenesis is normal. Torpor seems to play a larger part in the maintenance of obesity in fa/fa rats than in the mice mutants. In addition, inbred mouse strains such as NZO mice and Japanese KK mice are moderately obese. Certain hybrid mice, such as the Wellesley mouse, become spontaneously fat. Further, several desert rodents, such as the spiny mouse, do not become obese in their natural habitats, but do become so when fed on standard laboratory feed.
Animals which have been used as models for obesity have also been developed via physical or pharmacological methods. For example, bilateral lesions in the ventromedial hypothalamus (VMH) and ventrolateral hypothalamus (VLH) in the rat are associated, respectively, with hyperphagia and gross obesity and with aphagia, cachexia and anorexia. Further, it has been demonstrated that feeding monosodium-glutamate (MSG) or gold thioglucose to newborn mice also results in an obesity syndrome.
Each of the rodent obesity models is accompanied by alterations in carbohydrate metabolism resembling those in Type II diabetes in man. For example, from both ob and db, congenic C57BL/KS mice develop a severe diabetes with ultimate xcex2 cell necrosis and islet atrophy, resulting in a relative insulinopenia, while congenic C57BL/6J ob and db mice develop a transient insulin-resistant diabetes that is eventually compensated by xcex2 cell hypertrophy resembling human Type II diabetes.
With respect to ob and db mice, the phenotype of these mice resembles human obesity in ways other than the development of diabetes, in that the mutant mice eat more and expend less energy than do lean controls (as do obese humans). This phenotype is also quite similar to that seen in animals with lesions of the ventromedial hypothalamus, which suggests that both mutations may interfere with the ability to properly integrate or respond to nutritional information within the central nervous system. Support for this hypothesis comes from the results of parabiosis experiments (Coleman, D. L. 1973, Diabetologica 9:294-298) that suggest ob mice are deficient in a circulating satiety factor and that db mice are resistant to the effects of the ob factor. These experiments have led to the conclusion that obesity in these mutant mice may result from different defects in an afferent loop and/or integrative center of the postulated feedback mechanism that controls body composition.
In summary, therefore, obesity, which poses a major, worldwide health problem, represents a complex, highly heritable trait. Given the severity, prevalence and potential heterogeneity of such disorders, there exists a great need for the identification of those genes and gene products that participate in the control of body weight.
It is an objective of the invention to provide modulators of body weight, to provide methods for diagnosis of body weight disorders, to provide therapy for such disorders, and to provide assay systems for the screening of substances that can be used to control body weight.
The present invention relates to the discovery, identification and characterization of nucleotides that encode Ob receptor (ObR), a novel receptor protein that participates in the control of mammalian body weight. ObR, described for the first time herein, is a transmembrane protein that spans the cellular membrane once and is involved in signal transduction triggered by the binding of its natural ligand, ob, also known as leptin. ObR has amino acid sequence motifs found in the Class I cytokine receptor family, and is most related to the gp130 signal transducing component of the IL-6 receptor, the G-CSF receptor, and the LIF receptor. The results presented in the working examples herein demonstrate that a long-form ObR (predominantly expressed in the hypothalamus) transduces signal via a STAT mediated pathway typical of IL-6 type cytokine receptors, whereas a major naturally occurring truncated form or a mutant form found in obese db/db mice does not. The long form ObR can mediate activation of STAT proteins and stimulate transcription through IL-6 responsive gene elements. Reconstitution experiments indicate that, although ObR mediates intracellular signals with a specificity similar to IL-6 type cytokine receptors, signaling appears to be independent of the gp130 signal transducing component of the IL-6 type cytokine receptors.
The ObR mRNA transcript, which is about 5 kb long, is expressed in the choroid plexus, the hypothalamus and other tissues, including lung and liver. The murine short forms described herein encode receptor proteins of 894 (FIGS. 1A-1D) and 893 amino acids; murine long form obR cDNAs and human obR cDNAs, described herein, encode receptor proteins of 1162 amino acids and 1165 amino acids, respectively (FIGS. 1A-1D and FIGS. 3A-3F). The ObR has a typical hydrophobic leader sequence (about 22 amino acids long in both forms of murine ObR, and about 20 amino acids long in human ObR); an extracellular domain (about 815 amino acids long in both forms of murine ObR, and about 819 amino acids long in human ObR); a short transmembrane region (about 23 amino acids long in both forms of murine ObR and human ObR); and a cytoplasmic domain. The transcripts encoding the murine ObR short (FIGS. 1A-1D) and long form (FIGS. 6A-6F) are identical until the fifth codon 5xe2x80x2 of the stop codon of the short form and then diverge completely, suggestive of alternative splicing. As described herein, the cytoplasmic domain encoded by the 894 amino acid murine short form obR cDNA is 34 amino acids, while that encoded by the murine long form obR cDNA (302 amino acids) is approximately the same length as the cytoplasmic domain encoded by the human obR cDNA (303 amino acids). The deduced amino acid sequences from murine long form ObR and human ObR are homologous throughout the length of the coding region and share 75% identity (FIGS. 7A-7B).
The obese phenotype of the db mouse results from a Gxe2x86x92T transversion in the obR gene. This transversion creates a splice donor site which in turn leads to aberrant processing of obR long form mRNA in db mutants. In db mutants this aberrant processing generates long form mRNAs which encode a truncated ObR protein that is identical to the 894 amino acid short form ObR. Like the short form ObR, the mutant long form ObR lacks most of the cytoplasmic domain and is incapable of transducing a signal via a STAT mediated pathway. The signalling competant long form ObR, which is absent in the db/db mice, is required for body weight maintenance.
The invention encompasses the following nucleotides, host cells expressing such nucleotides, and the expression products of such nucleotides: (a) nucleotides that encode mammalian ObRs, including the human ObR, and the obR gene product; (b) nucleotides that encode portions of the ObR that correspond to its functional domains, and the polypeptide products specified by such nucleotide sequences, including but not limited to the extracellular domain (ECD), the transmembrane domain (TM), and the cytoplasmic domain (CD); (c) nucleotides that encode mutants of the ObR in which all or a part of one of the domains is deleted or altered, and the polypeptide products specified by such nucleotide sequences, including but not limited to soluble receptors in which all or a portion of the TM is deleted, and nonfunctional receptors in which all or a portion of the CD is deleted; (d) nucleotides that encode fusion proteins containing the ObR or one of its domains (e.g., the extracellular domain) fused to another polypeptide.
The invention also encompasses agonists and antagonists of ObR, including small molecules, large molecules, mutant Ob proteins that compete with native Ob, and antibodies, as well as nucleotide sequences that can be used to inhibit obR gene expression (e.g., antisense and ribozyme molecules, and gene or regulatory sequence replacement constructs) or to enhance obR gene expression (e.g., expression constructs that place the obR gene under the control of a strong promoter system), and transgenic animals that express an obR transgene or xe2x80x9cknock-outsxe2x80x9d that do not express ObR.
In addition, the present invention encompasses methods and compositions for the diagnostic evaluation, typing and prognosis of body weight disorders, including obesity and cachexia, and for the identification of subjects having a predisposition to such conditions. For example, obR nucleic acid molecules of the invention can be used as diagnostic hybridization probes or as primers for diagnostic PCR analysis for the identification of obR gene mutations, allelic variations and regulatory defects in the obR gene. The present invention further provides for diagnostic kits for the practice of such methods.
Further, the present invention also relates to methods for the use of the obR gene and/or obR gene products for the identification of compounds which modulate, i.e., act as agonists or antagonists, of obR gene expression and or obR gene product activity. Such compounds can be used as agents to control body weight and, in particular, as therapeutic agents for the treatment of body weight and body weight disorders, including obesity, cachexia and anorexia.
Still further, the invention encompasses methods and compositions for the treatment of body weight disorders, including obesity, cachexia, and anorexia. Such methods and compositions are capable of modulating the level of obR gene expression and/or the level of obR gene product activity.
This invention is based, in part, on the surprising discovery, after an extensive survey of numerous cell lines and tissues, of a high affinity receptor for ObR in the choroid plexus of the brain, the identification and cloning of obR cDNA from a library prepared from choroid plexus mRNA, characterization of its novel sequence, mapping the obR gene to the same genetic interval in the mouse genome as the db gene maps, and characterization of the ObR as a transmembrane receptor of the Class I cytokine receptor family. obR mRNA was detected in other tissues, including the hypothalamus.
The full-length ObR, expressed predominantly in the hypothalamus signals transduces through activation of STAT proteins and stimulation of transcription through IL-6 responsive gene elements. The ability of the full-length long form ObR to signal is in contrast to the naturally occurring truncated form or the mutant form found in db/db mice which are unable to mediate signal transduction. The invention also includes forms of ObR lacking one or another of the intracellular domains important for signalling and induction of gene expression.
In another aspect the invention features a method for identifying candidate therapeutic agents for the treatment of a body weight disorder, comprising:
a) providing a cell which expresses a mammalian ob receptor, the cell harboring a reporter construct, the reporter construct including a sequence encoding a detectable protein (e.g., xcex2-lactamase, chloramphenicol acetyltransferase (CAT), adenosine deaminase (ADA), aminoglycoside phosphotransferase (neor, G418r), dihydrofolate reductase (DHFR), hygromycin-B-phosphotransferase (HPH), thymidine kinase (TK), lacZ (encoding xcex2-galactosidase), alkaline phosphatase or xanthine guaninephosphoribosyltransferase (XGPRT)), the sequence encoding the detectable protein being operably linked to a Ob receptor responsive regulatory element (e.g., IL-6 RE or HRRE);
b) contacting the cell with a test compound;
c) measuring the expression of the detectable protein in the presence of the test compound;
d) wherein an increase or a decrease in the expression of the detectable protein in the presence of the test compound compared to the absence of the test compound indicates that the test compound is a candidate therapeutic agent for treatment of a body weight disorder.
The sequence encoding the detectable protein is operably linked to the Ob receptor responsive regulatory element if expression of the protein is altered by activation of the Ob receptor (e.g., activation caused by the binding of leptin to the Ob receptor).
In other embodiments the contacting with the test compound takes place in the presence of an Ob receptor agonist or antagonist.
The invention also features a compound identified using the method described above. In another aspect the invention features a method of inducing weight loss in a mammal by administering to the mammal a compound (e.g., a small molecule or an antibody) that activates the Ob receptor or the Ob receptor signalling pathway.
As used herein, the following terms, whether used in the singular or plural, will have the meanings indicated:
Ob: means the Ob protein described in Zhang, Y. et al., 1994, Nature 372:425-432, which is incorporated herein by reference in its entirety, which is also known as leptin. Ob includes molecules that are homologous to Ob or which bind to ObR. Ob fusion proteins having an N-terminal alkaline phosphatase domain are referred to herein as AP-Ob fusion proteins, while Ob fusion proteins having a C-terminal alkaline phosphatase domain are referred to herein as Ob-AP fusion proteins.
obR nucleotides or coding sequences: means nucleotide sequences encoding ObR protein, polypeptide or peptide fragments of ObR protein, or ObR fusion proteins. obR nucleotide sequences encompass DNA, including genomic DNA (e.g. the obR gene) or cDNA, or RNA.
ObR: means Ob receptor protein. Polypeptides or peptide fragments of ObR protein are referred to as ObR polypeptides or ObR peptides. Fusions of ObR, or ObR polypeptides or peptide fragments to an unrelated protein are referred to herein as ObR fusion proteins.
A functional ObR refers to a protein which binds Ob with high affinity in vivo or in vitro.
ECD: means xe2x80x9cextracellular domainxe2x80x9d.
TM: means xe2x80x9ctransmembrane domainxe2x80x9d.
CD: means xe2x80x9ccytoplasmic domainxe2x80x9d.